The Hampden District Attorney's Office is asking a federal judge to dismiss a civil suit claiming that prosecutors failed to properly investigate an alleged 2015 police beating in Springfield.

The lawsuit claims that Jackie Ligon, Jozelle Ligon and Michael Cintron had their civil rights violated when a group of men -- allegedly including off-duty Springfield police officers -- assaulted them in the early morning hours of April 8, 2015.

The Ligon brothers, Cintron and their cousin Paul Cumby were allegedly beaten by a group of men in parking lot near Nathan Bill's Bar and Grill after arguing in the bar with a group of off-duty Springfield cops. Cumby filed a complaint to the department regarding the assault and the alleged failure of officers who responded to the fight to search for the assailants.

That complaint led to both criminal and internal investigations by the department, as well as disciplinary charges against 12 Springfield officers which remain unresolved. Hampden County District Attorney Anthony Gulluni reviewed the case and declined to press charges, saying that while the men were clearly victims of an assault, the identifications they gave of their attackers were too weak and inconsistent to build a case on.

Both the FBI and Massachusetts Attorney General's Office took up the case, continuing a criminal investigation into the attack. The AG's office has subpoenaed records and brought witnesses before an ongoing grand jury, which could lead to indictments against Springfield officers.

The lawsuit filed by Cintron and the Ligons names as defendants the city of Springfield, its police department, Police Commissioner John Barbieri and over a dozen individual officers.

It also targets the Hampden DA's office, alleging that prosecutors conspired with the police department to undermine the investigation and avoid bringing charges.

In its response to the suit, the DA's office denied the allegations and argued that it is immune from civil lawsuits about prosecutorial decisions.

"Even assuming the allegations are true -- which they are not -- the claims must be dismissed," DA's attorneys wrote in the motion to dismiss.

The motion also claims that the victims have no standing to sue over a non-prosecution and that their suit alleges no plausible conspiracy to violate their civil rights. If the claims against the DA are dismissed, it does not mean the end of the lawsuit; the city and police defendants have not yet responded to the allegations.

Joe Smith III, the plaintiffs' attorneys, told MassLive that the DA's decision to prosecute was not at issue. Rather, the civil rights violations stemmed from deliberate collusion between the DA's office and the police department, Smith said.

"They conducted an investigation in which they used the same officer that used an identification process that deviated from the norm in an effort to confuse my clients," Smith wrote in an email. "They conspired to help the officers avoid criminal prosecution."

In their motion to dismiss, Gulluni and the DA's office argue both that there was no conspiracy and that the lawsuit improperly targets the decision not to bring charges.

"These allegations can only be read to mean that a proper investigation would have established probable cause which would have led the DA Defendants to prosecute the Defendant Police Officers," the motion says. "It has long been established, however, that a prosecutor's decision to prosecute or not prosecute is not cognizable harm to a private citizen who is not the target of the prosecution."

The motion also asks the judge to dismiss the individual claims against Gulluni and First Assistant DA Jennifer Fitzgerald, saying that they have "absolute immunity" from lawsuits related to their work as prosecutors.

Daniel Medwed, a professor at Northeastern University School of Law, said that immunity doctrines are well established law and make it "extremely difficult" to successfully sue procesutors.

"Suing prosecutors in federal court is like scoring a touchdown on a Hail Mary pass when you have eight men on the field," Medwed said.

Individual prosecutors have absolutely immunity for actions taken during litigation and further immunity for any investigative actions taken in good faith. And suing a DA's office as a whole is difficult as well, though can be successful if a suit proves a pattern of unconstitutional conduct, Medwed said.

"The rationale is you don't want prosecutors to be timid," he said. "To be scared of make tough calls for fear of being sued for damages."

Medwed noted that parts of the motion may have been written for a broader audience than the judge hearing the case.

Motions to dismiss must be written assuming that all the facts alleged in the lawsuit are true, and argue that the suit should be dropped regardless. But the DA's motion goes further, describing parts of the complaint as "grossly inaccurate" and denying the claims.

That language may not impress a judge, but could prove useful in a case that is expected to receive public and political scrutiny, Medwed said.

"Prosecutors declining to prosecute cases involving police violence, that's a political lighting rod," Medwed said. "I wasn't surprised to see some of the language in the motion to be a bit populist -- to go beyond the four corners of a legal argument but making a kind of a political appeal to justify the decision."